Cardiovascular System Notes

Cardiac Cycle

  • Systole and diastole.
  • Phase 1: Atrial Ventricular Diastole
    • All four chambers are relaxed and passively filling with blood.
    • AV valves are open, allowing blood to flow from atria to ventricles.
    • Deoxygenated blood from the systemic circulation enters the right atrium via the vena cava.
    • Oxygenated blood from the lungs enters the left atrium.
  • Atrial Systole
    • Atria contract to push the last bits of blood into ventricles, which are still in diastole.
  • Ventricular Systole
    • Ventricles contract while atria fill with blood.
    • AV valves are closed to prevent backflow.
    • Semilunar valves are forced open by ventricular contraction.
  • Cardiac output should be approximately 5 liters per minute if all goes well.

Cardiac Output

  • Definition: The amount of blood coming out of the left ventricle per minute.
  • Indicator of heart function.
  • Calculation: Cardiac Output = Heart Rate \, \times \, Stroke Volume
    • Heart Rate: Number of heartbeats per minute (approximately 70 bpm at rest).
    • Stroke Volume: Amount of blood ejected by the left ventricle in each beat (approximately 75 mL).
    • Result: 70 \, beats/min \, \times \, 75 \, mL/beat \approx 5 \, L/min
  • Cardiac output calculations will be extended later with calculations at rest and exercise.

Electrical Activity of the Heart

  • Similar to the nervous system and skeletal muscle regarding depolarization.
  • Electrical events lead to mechanical events (muscle contraction and relaxation).
  • Key Difference: Cardiac muscle is involuntary, unlike voluntary skeletal muscle.
    • Heartbeats occur without conscious input.
  • Smooth Muscle: Present in blood vessels (to be discussed).

Autoerythmusity: Special Capacity of Cardiac Muscle Cells

  • Some cardiac muscle cells (small number, about 1%) can generate their own action potentials (autorhythmic potentials).
  • These cells undergo regular cycles of depolarization and repolarization without external input.
  • These special cells are self-excitable.
  • Pacemaker Cells: Special cells responsible for autoerythmusity.
    • Make up approximately 1% of heart cells.
    • Other 99% of cells are non-pacemaker cells (cardiac myocytes).
  • Connection via Gap Junctions
    • Non-pacemaker cells are connected to pacemaker cells via gap junctions.
    • Gap junctions allow movement of ions between cells.
    • Depolarization in pacemaker cells spreads to non-pacemaker cells, causing them to depolarize.
  • Heart's Independence: Enables the heart to function independently of the nervous system.

Types of Pacemaker Cells

  • Sinoatrial (SA) Node: Located in the right atrium.
  • Atrioventricular (AV) Node: Located at the junction between the atria and ventricles.
  • Bundle of His: Left and right bundles that go between the ventricles.
  • Purkinje Fibers: Spread around the two ventricles.

Electric Current Propagation

  • SA node is the master pacemaker because it is the fastest.
  • AV node, bundle of His, and Purkinje fibers respond to the SA node in a normal heart.
  • SA Node Depolarization Rate
    • If SA node depolarizes 70 times a minute, the heart beats 70 times a minute.
    • If SA node depolarizes 90 times a minute, the heart rate increases.
  • Pathways of Electric Current
    • Interatrial Pathway: Message from right atrium to left atrium.
    • Internodal Pathway: Message from SA node to AV node.
  • Events Following Electric Current
    • Atrial contraction due to depolarization of SA node and interatrial pathway.
    • Message from AV node travels through bundle of His and Purkinje fibers, causing ventricular contraction.
    • Cycle repeats approximately 70 times per minute at rest.

AV Node Delay

  • It is important to delay electric current propagation at AV node.
  • AV node delay corresponds to the phase in the cardiac cycle when the atria complete emptying blood into the ventricles.
  • Without AV node delay, all four chambers would contract simultaneously, leading to inefficient blood flow.

Control of Heart Rate

  • Though heart can function independently, two systems control the sinoatrial node and heart rate when the heart is intact in the body.
  • Nervous System
    • Parasympathetic Nervous System: Slows down the heart rate.
      • Reduces SA node depolarization.
      • Maintains heart rate at approximately 70 bpm at rest.
    • Sympathetic Nervous System: Increases heart rate.
      • Increases SA node depolarization.
      • Occurs during exercise, raising heart rate to approximately 100 bpm or more.
  • Endocrine System: Hormones like adrenaline can control SA node and heart rate.

Factors Influencing Heart Rate

  • Body Temperature: Increased temperature leads to increased heart rate.
    • Speeds up blood flow and facilitates heat dissipation.

Key Concepts for Understanding the Heart

  • Types of circulatory systems.
  • Unidirectional flow of blood.
  • Four valves.
  • Cardiac cycle.
  • Electrical system driving mechanical events.

Blood Vessels

  • Theme: Structure relates to function.
  • Three Types of Blood Vessels: Arteries, veins, capillaries.
  • Diameter of circulatory system affects velocity and blood pressure.
  • Blood pressure measurement and readings in various parts of the body.
  • Function of capillaries.
  • Brief overview of the lymphatic system.

Function of Circulatory System

  • Ultimate Function: Delivery of gases, nutrients, transport of carbon dioxide and waste.
  • Transport and Exchange: Key processes in the circulatory system.
  • Heart's Role: Generates pressure for blood flow (left ventricle).
  • Blood Vessels Role: Ensure contact with each and every cell of the body for oxygen, nutrients, and waste exchange.

Infrastructure of Circulatory System

  • Heart ejects oxygenated blood from the left ventricle into the aorta.
  • Aorta carries blood to arteries.
  • Arteries branch into arterioles.
  • Arterioles lead to capillaries, for the exchange of substances.
  • Capillaries: Oxygen and nutrients leave, carbon dioxide and wastes enter.
  • Deoxygenated blood collected by venules.
  • Venules merge into veins.
  • Veins carry blood to the vena cava, which empties into the right atrium.
  • Cycle Repeats: Right atrium, right ventricle, lungs, left atrium, left ventricle, aorta, arteries, arterioles, capillaries, venules, veins, vena cava, right atrium.

Three Major Types of Blood Vessels

  • Arteries
  • Veins
  • Capillaries
  • Arterioles and venules have similar structure and function to arteries and veins, respectively.

Structure of Arteries and Veins

  • They have three layers:
    • Endothelial Layer (Innermost Layer)
      • Functions: Produces substances that dictate the diameter of the blood vessel.
      • Regulates contraction and relaxation of smooth muscle.
    • Smooth Muscle Layer (Middle Layer)
      • Functions: Capable of changing the diameter of blood vessels.
      • Contraction: Reduces the diameter.
      • Relaxation: Opens up the diameter.
    • Connective Tissue Layer (Outermost Layer)
      • Functions: Produces proteins that are important for the structural support of the blood vessels.
      • Example: Collagen.
  • Smooth muscle layer is thicker in arteries compared to veins.
  • Arteries need to have a thick wall because they need to withstand pressure coming from the left ventricle.

Veins

  • They do not deal with too much pressure.
  • They have the problem of working against gravity.
  • They use valves to prevent the backflow of blood.

Capillaries

  • Their function is exchange.
  • They have one layer of cells (endothelial layer) and a tiny basement membrane.
  • Basement membrane provides support for the blood vessels.

Arteries

  • Problem: The pressure from the left ventricle.
  • They have a thick wall.
  • Elasticity: Vessels can stretch and take up blood.

Systole vs Diastole

  • Heart Contracting and Emptying - Left Ventricular Systole
  • Heart Relaxing and Filling - Left Ventricular Diastole
  • Left Ventricle During Systole
    • High pressure. Roughly 120 mm mercury.
  • Left Ventricle During Diastole
    • Zero pressure.

Arteries Pressure

  • Systemic circulation at all times at a good pressure.
  • Left Ventricle During Systole: 120 mm mercury pressure.
  • Left Ventricle During Diastole: 70-80 mm mercury pressure.

Veins Function

  • Bring blood back to the heart to work against gravity.
  • They rely on some skeletal muscle activity.
    • Skeletal muscle contracts to squeeze blood in the veins.
    • The blood moves until skeletal muscle activity relaxes and valves close to prevent backflow of blood.

Venous Return & Edema

  • Venous Return - Blood coming back from veins (vena cava). Back to the right side of the heart.
  • Not Enough Skeletal Muscle Activity
    • The blood starts building up.
    • Fluid starts accumulating = edema.
  • Pulmonary Edema: Blood starts building up in lungs due to the left heart failure.
  • Peripheral Edema: If veins don't do their job, you get peripheral edema.

Capillaries Structure

  • Simple structure.
  • One layer of cells, the endothelial layer.
  • They leave spaces between the pores.
  • Fluids can be transported. Water passes in and out.
  • The outside is the interstitial fluid.
  • Capillaries: Time when things can leave and enter into interstitial fluid.

Capillary Exchange

  • Lipid-soluble substance: Can go through the cell membrane of the endothelial cells.
  • Oxygen - Lots of blood oxygen, so makes it easier to leave.
  • Carbon Dioxide - Little partial pressure in plasma, so carbon dioxide comes into circulatory system.